Cytokines, such as interleukins and thrombopoietin, control the development and physiology of hematopoietic lineages and aberrant cytokine signalling has been associated with the initiation and/or progression of leukemias. Upon binding of cytokines to their respective receptors several signalling pathways are activated that elicit various cellular responses, including proliferation, differentiation, survival and death. The majority of cytokines activate pathways that utilize the transcription factors Stat5a and Stat5b (Stat5). In fact, aberrant activation of Stat5 has been observed in leukemias, which suggests that it plays a critical role in disease progression. Deletion of the transcription factor Stat5 from the mouse genome results in multiple defects in many organ systems, emphasizing a crucial role of cytokines in a number of different cell types. Researchers in LGP in collaboration with other scientists have used mice from which the Stat5 genes had been deleted either from the germline or from specific cell types to explore the mechanisms by which Stat5 controls normal and neoplastic hematopoiesis. Our findings demonstrated a critical role of Stat5 in the function of the immune system (B and T cells) and in some myeloid disorders. [unreadable] [unreadable] Recent findings demonstrated that the transcription factor Stat5 is essential for the development of myeloproliferative disease induced by the TEL-PDGFRB translocation, wich suggests that inhibitors against Stat5 might be suitable approach to treat various forms of leukemias. Other findings demonstrated that Stat5 is critical for early B cell development and accurate generation of T helper cells.